The invention relates to a process for the production of oxygen and nitrogen by low-temperature (cryogenic) separation of air in a rectification system that has a high-pressure column and a low-pressure column, whereby in the process, charging air is introduced into the high-pressure column, an oxygen-containing liquid fraction is removed from the high-pressure column and fed into the low-pressure column, gaseous nitrogen from the low-pressure column is at least partially condensed in a top condenser by indirect heat exchange with an evaporating cooling fluid, a nitrogen product stream is removed from the low-pressure column and/or the high-pressure column, and an oxygen product stream is pulled off from the low-pressure column.
Processes with top cooling of the low-pressure column are known from EP 1022530 A1, WO 9819122 or EP 811816 A2. A process of the type mentioned above is shown in EP 955509 A1.
An object of the invention is to provide a process of the type mentioned above, as well as corresponding apparatus therefor, which works especially efficiently.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
These objects are achieved through the use of an intermediate liquid, that is drawn from an intermediate point on the low-pressure column, as the cooling fluid for the top condenser of the low-pressure column.
In EP 955509 A1, the cooling liquid for the top condenser of the low-pressure column is formed from the bottom liquid of the high-pressure column. In comparison to this, the invention has two advantages. On the one hand, the composition of the cooling fluid can be selected such that the product yield, especially the oxygen yield, is increased. On the other hand, the cooling fluid has a lower pressure, so that before its introduction into the top condenser, it has to be throttled only relatively slightly (e.g., 1.7 to 3 bar); the flash gas losses are correspondingly smaller.
To optimize the oxygen yield, it is advantageous if the intermediate liquid, which is used as cooling fluid in the top condenser, is drawn off from an intermediate point that lies above the point at which the oxygen-containing liquid fraction is fed from the high-pressure column into the low-pressure column. For example, 10 to 20 theoretical plates or 12 to 25 actual plates lie between them.
In the process, liquid nitrogen can be drawn off from the low-pressure column or from its top condenser and can be pulled off as liquid nitrogen product. In particular in this case, the process preferably has a nitrogen circuit. In this case, the circuit nitrogen is drawn off in gaseous form from the upper area of the high-pressure column and is compressed in a circuit compressor, whereby a first partial stream of the compressed circuit nitrogen is actively depressurized, and a second partial stream of the compressed circuit nitrogen is liquefied and fed back into the rectification system and/or is removed as another liquid nitrogen product. Such a circuit can also be used without oxygen production, for example with use of the bottom liquid of the low-pressure column or high-pressure column as a cooling fluid in the top condenser of the low-pressure column.
Cold is produced by the active depressurization of the first partial stream of the circuit nitrogen, and said cold is further transported via the liquefied second partial stream and can be used to increase the liquid production. The liquid from the circuit can be drawn off directly, for example, as liquid nitrogen product. As an alternative or in addition, the liquefied circuit nitrogen can be fed to one of the columns of the rectification system, preferably the high-pressure column. The corresponding amount can then be drawn off from the rectification system as liquid nitrogen and/or oxygen product.
Top nitrogen of the high-pressure column is liquefied in a main condenser. The condensate that is produced in this case is released at least in part, preferably for the most part, as reflux to the high-pressure column. Another portion of the condensate can be introduced into the low-pressure column above and/or drawn off immediately as liquid nitrogen product.
The circuit can be designed as a two-turbine or multi-turbine system by having a third partial stream of the compressed circuit nitrogen be actively depressurized independently of the first partial stream.
In addition, the invention relates to a device according to an apparatus for production of oxygen and nitrogen by low-temperature (cryogenic) separation of air comprising:
(a) a rectification system having a high-pressure column (4) and a low-pressure column (5),
(b) (b) a feed line (1, 3) for introducing feed air into the high-pressure column (4),
(c) (c) a liquid line (8, 10) for transferring an oxygen-containing liquid fraction from the high-pressure column (4) into the low-pressure column (5),
(d) (d) a top condenser (7) having a liquefaction chamber which is in fluid communication (17, 18) with the upper section of said low-pressure column (5) and the top condenser having an evaporation chamber which is in fluid communication with a coolant line (15) for introducing a cooling fluid,
(e) a nitrogen-product line (19) which is connected to the low-pressure column (5) and/or the high-pressure column (4), and
(f) an oxygen-product line (61, 62, 63) which is connected to the low-pressure column (5), wherein the coolant line (15) is connected to an intermediate point (15) on the low-pressure column (5).